Present approaches in focal-plane video imaging systems use some form of analog multiplexing of the pixel data in order to read the image information. It is this multiplexing which defines the so called video data rates. In broadcast television, for example, the 30 hertz pixel data rate is multiplexed to the 4 megahertz video data rate. This same situation exists in industrial and military video systems where pixel rates are usually below 3 kilohertz and analog multiplexing is used with resulting megahertz video rates.
These multiplexing approaches have necessitated the use of analog to digital conversion processes employing high speed circuitry which, as a practical matter cannot be readily integrated with a focal-plane sensor. Moreover, the typical A/D converter in these applications comprises a high-speed video, flash converter which is generally considered too expensive for use in consumer applications.
As is discussed in Oversampling Delta-Sigma Data Converters, edited by James C. Candy and Gabor C. Temes, IEEE Press, 1992, New York, oversampled analog to digital (A/D) converters are known which use coarse quantization at a high sampling rate combined with negative feedback and digital filtering to achieve increased resolution at a lower sampling rate.
Such converters may, therefore, exploit the speed and density advantages of modern very large scale integration (VLSI) while at the same time reducing the requirements for component accuracy.
In a type of oversampled A/D converter generally known as a delta-sigma modulator, the analog input is sampled at a rate well above the Nyquist frequency and fed to a quantizer via an integrator. The quantized output is fed back and subtracted from the input. This feedback forces the average value of the quantized output to track the average analog input value.